Pseudomonas aeruginosa tryptophan synthase is an unusual biosynthetic operon regulated by substrate induction rather than end product repression. Preliminary genetic studies indicate that expression of the structural genes for the two enzyme subunits depends on a closely linked regulatory gene with a diffusible product that activates transcription in the presence of effector molecules. There is suggestive evidence that this regulatory gene product also represses expression somewhat in the absence of the inducer. This project aims to confirm and quantify this activation-induction model by in vitro experiments and to determine the DNA sequence of the structural and regulatory elements. It will also investigate the nature of constitutive mutants in the system. Just as the fluorescent pseudomonads have evolved a novel regulatory system for tryptophan synthase, so have the enzymes structural genes evolved so that they cannot interact productively with subunits from other bacteria. Mutational analysis will be used to identify the specific regions of the molecule involved in this discrimination. This should identify a subset of residues likely not previously recognized as crtical to enzyme function.